Photodetectors (or photosensors) are sensors capable of detecting light or other electromagnetic energy. This is accomplished by utilizing the interband transition of electrons in a quantum well to detect an energy level of a photon. A quantum well has subbands and when an electron in the quantum well is excited by a photon, the electron jumps to another subband according to the energy level of the photon. When an energy level of a photon is greater than the gap between subband energy levels and the photon enters a quantum well of a photodetector, the electrons in the quantum well become excited and move to an upper subband. In some cases, this movement of excited electrons to the upper subband is described as “tunneling the barrier.” Such electron transition causes an electric current through the photodetector.
As nano-photonic devices are increasingly used in various applications (e.g., compact digital cameras), photodetectors for those applications have become smaller than the wavelength of the light to be detected. In this case, the detection efficiency drops very rapidly because the dielectric waveguide structure is inefficient in the sub-wavelength region.